What do 8-bit and 12-bit grayscale mean and which should I use when scanning?

TECHNO BYTES Q & A

What do 8-bit and 12-bit grayscale mean andwhich should I use when scanning?Demetrios J. Halazonetis

Grayscale images have pixels that range fromblack to white, going through a number ofintensity steps in between. Most images allow

256 different intensity levels, a number that arises fromthe 256 different values that a byte can take. Because abyte is composed of 8 bits, such images are known as8-bit grayscale images. Some high-end scanners canscan with a finer intensity scale. They use 12 or 16 bitsto represent the intensity values of the image, making itpossible to register 4096 or 65,536 different gray levels.The number of bits used for intensity values is knownas the bit depth. Although a high bit depth seems apreferred option, in practice, the benefits are limited.Here are some points to keep in mind about 12-bit or16-bit images:

● They will take up more space.● They can be saved in limited file formats.● Not all imaging software can work with 12-bit or

16-bit pictures. Even if they can, their functionalitymight be reduced.

● Not all cephalometric software can work with 12-bitor 16-bit images. You might have to reduce theimage to 8 bits before digitizing.

● You will see no difference on your computer screen,because computer monitors are set up to show onlyup to 256 shades of gray. Even if the monitor couldshow you more (this is possible by using some fancyprogramming tricks), the human eye cannot discernso many.

● You will not see the dark areas of the image better.The main problem in scanning radiographs is todiscern detail in the dark areas (eg, under the chinaround the hyoid bone and neck). This problem will

Assistant professor, Orthodontic Department, University of Athens DentalSchool, Athens, Greece.Reprint requests to: D. Halazonetis, 6 Menandrou St, Kifissia 145 61, Greece;e-mail, dhal@dhal.com.Submitted and accepted, July 2004.Am J Orthod Dentofacial Orthop 2005;127:387-80889-5406/$30.00Copyright © 2005 by the American Association of Orthodontists.

not be solved by increasing the bit depth because thelimitation is based on the optical density of thescanner and the electronics.

So what good is higher bit depth? If you are notgoing to manipulate the image severely, then scanningat 16-bits is not recommended. On the other hand, ifyou need to change contrast and brightness to a ratherextreme extent, it is best to work with high bit-depthimages and convert at the end. To gain a personalfeeling on what “extreme” is, I spent a whole afternoonscanning radiographs and changing them in Photoshopto see whether differences were appreciable. I managedto construct an example that demonstrates a worthwhileadvantage of 16-bit scanning. Figure 1 shows an initialscanned image, after applying a gamma of 0.5 to makeit even darker, and then applying a gamma of 6 to makeit lighter; A is the 8-bit scan, and B is the 16-bit scan.Notice the posterization of the 8-bit image and thecomplete loss of the lower mandibular border andmandibular canal.

A comparison of the histograms of the final imagesis even more impressive. The histogram in Figure 2, Ashows that the 8-bit image has been completely de-pleted of gray levels in the dark range, showing largegaps, whereas the 16-bit image histogram (Fig 2, B) iscontinuous.

Images derived from drum scanners that achievehigh optical densities and images from medical digitalequipment (eg, CT scans) might need more than 8 bitsto represent the wide range of intensities that areregistered. In such cases, the user can select a subset ofthe total intensity range. Figure 3, A shows a CT-scanimage (http://www.barre.nom.fr/medical/index.html)and its histogram. The image is 16-bit, but the usefulrange extends across only 1900 different intensitylevels of the possible 65,536. The histogram (Fig 3, B)shows the useful part of the intensity range. Note the 3distinct parts of the histogram: extreme left, center, and

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treated as above. Note detail in lower mandibular border and mandibular canal.

American Journal of Orthodontics and Dentofacial OrthopedicsMarch 2005

388 Techno bytes Q & A

Fig 2. Histograms of final images in Figure 1.

Fig 3. CT-scan image (http://www.barre.nom.fr/medical/

Fig 4. Histogram of image in Figure 3 and subimages extracted from each

Fig 1. A, 8-bit scan showing (left to right) initial scanned image, after applying gamma of 0.5 tomake it darker, and then applying gamma of 6 to make it lighter. Notice posterization. B, 16-bit scan,

of 3 intensity ranges. Histogram extends over 1900 intensity levels.